In offset lithography, an image is present on a plate as a pattern of ink-accepting and ink-repellent surface areas. In a typical sheet-fed offset press system, the plate is mounted to a plate cylinder, is inked and makes contact with the compliant surface of a blanket cylinder which, in turn, applies the image to paper sheets pinned to an impression cylinder, which brings the sheets into contact with the blanket cylinder.
Traditionally, the plates for an offset press have usually been produced photographically. More recently, however, a number of electronic alternatives have been developed some of which can be used on-press. With such systems, digitally controlled imaging devices alter the ink-receptivity of a plate blank mounted to a plate cylinder in a pattern representative of the image to be printed. Such imaging devices include lasers that cause chemical changes on the plate blank, ink jets that directly deposit ink-repellent or ink-accepting spots on the plate blank and spark or ion discharge devices which physically alter the topology of the plate blank. These various methods of imaging lithographic plates are described in detail in U.S. Pat. Nos. 3,506,779; 4,054,094; 4,347,785; 4,911,075 and 5,385,092, among others.
Another approach to imaging lithographic plates described, for example, in U.S. Pat. Nos. 3,945,318; 3,962,513 and 4,395,946 involves the use of thermal-transfer materials. In these systems, a plastic acceptor sheet or plate is mounted to a cylinder and a so-called donor sheet, coated with a transferable material, is wrapped around the acceptor sheet with the transferable material facing the acceptor sheet. To create an image, the cylinder is rotated and a digitally controlled laser scans the donor/acceptor sheet set so that the transfer material is selectively irradiated through the donor sheet. The irradiation causes the transfer material to adhere preferentially to the acceptor sheet. The transfer material and acceptor sheet material have different affinities for ink so that when the donor sheet is removed from the cylinder, the acceptor sheet is left with an image pattern on its outer surface.
The donor sheet could also be a so-called strip sheet which is coated with a layer of an adhesive and laid over an ink-receptive acceptor sheet or plate coated with an ink-repellent material such as silicone. In this case, imaging may be accomplished by projecting laser energy through the strip sheet onto the plate. When the strip sheet is removed, the adhesive will pull the imaged silicone areas away from the plate, while leaving the remaining silicone attached to the plate thus forming the image.
When mounting an imaged plate to a plate cylinder for a press run or when mounting a plate blank to a cylinder for imaging or platemaking, it is essential that the plate be wrapped tightly around the cylinder and that the head and tail end of the plate be secured firmly to the cylinder so that when the cylinder is rotated, there will be no relative movement between the plate and the cylinder.
Likewise, when a donor/acceptor sheet set is mounted to a cylinder for platemaking by thermal transfer, both sheets must be firmly clamped to the plate to avoid all relative movement.
Various devices have been developed over the years for holding a lithographic plate to a plate cylinder. These devices have run the gamut from vacuum clamps to assorted mechanical and electromechanical clamps. For the most part, they have all tended to be relatively complex and costly. Also, in many cases, different clamping mechanisms are required to secure the head and tail ends of the plate to the cylinder. In addition, some prior clamping devices have to be repositioned on the cylinder in order to secure different length plates to the cylinder.
Accordingly, it would be advantageous if there did exist a mechanism for mounting a lithographic plate or donor/acceptor sheet set to a cylinder which would avoid the above problems.